08-07-2008, 03:06 PM
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That depends on your needs. If you are experiencing diurnal changes in your pH levels, then by adding a light, timed opposite your display lights, will help to reduce or eliminate those swings. The diurnal changes in pH are common in reef aquaria and are result of changes in the level of CO2 in the tank. During the daylight hours, the process of photosynthesis converts CO2 into carbohydrate and oxygen. The lack of CO2 results in a higher pH. The respiration of organisms within your tank is constantly adding to the CO2 in the system, so at night, the CO2 levels rise, effectively reducing the pH. By adding the light to the sump at night, you allow the process of photosynthesis to continue in your refugium life throughout the nighttime hours, thereby continuing to keep the CO2 levels down, and subsequently keeping pH levels regulated.
If you have little in the way of diurnal pH swings, and are more concerned with the nutrient export performed by macro algae growth, then you might seek to keep the light on 24hrs.
and finally, if you have a sump with no refugium, then you have little if any need for a light on your sump. Except maybe to see to do your cleaning and maintenance.
Hope that Helps.